The aim of this proposal is to establish the structure of apolipoprotein CI from human plasma. Since little information is available about 3D structures of this class of macromolecules, we feel that apoCI, as the smallest member of this class, will serve as a structural paradigm. We have crystallized and attempted to solve the structure of apoCI by MIR methods. Native crystals diffract to 2.0 E resolution with conventional x-ray sources. Because of the small size of the protein (57 residue), we failed to find an appropriate isomorphous derivative. Our ability to generate selenomethionine-containing recombinant proteins, coupled with our previous experience with data collection at SSRL (BL1-5AD) on selenomethionine-substituted apolipoprotein E3 provides a solid basis for completion of this project.